In Japanese Patent Applications Nos. 2003-320884 and 2005-199939, a seat suspension as follows is disclosed. Specifically, an upper frame provided to be vertically movable with respect to a lower frame attached to a vehicle body floor is supported elastically with a magnetic spring and a torsion bar. Utilizing that the magnetic spring has a negative spring constant in a predetermined displacement range, by combining with the torsion bar having a positive spring constant, a spring constant resulted from superposing both the springs in the predetermined displacement range becomes substantially zero, making a structure such that even when a friction force of a link or the like is included therein, the spring constant (stationary spring constant) of the entire seat suspension (entire system) is kept in the vicinity of the above-described substantially zero, so as to absorb vibrations. Further, Japanese Patent Application No. 2010-179719 has proposed a technology to elastically support a fixed magnet unit constituting the magnet spring on an upper frame side or a lower frame side, so as to further improve vibration damping performance.
On the other hand, Japanese Patent Application No. H10-203221 discloses a seat suspension in which six hydraulic cylinders are disposed to actively control a seat in six-axis directions.